The disclosure relates to treads for tires and more particularly to the designs of the tread pattern for these treads and to the tires provided with such treads which have a lasting ability to drain away standing water present on the roadway in times of wet weather, these treads having improved wear performance.
As is known, wet weather driving conditions require the most rapid possible elimination of the water from the contact patch in which each tire is in contact with the roadway so as to ensure that the tread makes contact with the roadway. Water that is not pushed over the front of the tire flows partially along the grooves and sipes formed in the tire tread, whether this be in the circumferential or transverse direction.
A cut refers generally either to a groove or to a sipe and corresponds to the space delimited by walls of material facing one another and distant from one another by a non-zero distance (referred to as the “width of the cut”).
A groove here means a cut that opens onto a tread surface intended to be in contact with the roadway, this cut having a mean width such that the walls of material delimiting it are never in contact with one another under the normal service conditions of the tire. The grooves may have any shape when viewed in cross section and in terms of their path along the tread surface and may be oriented in any direction. The path of a groove along the tread surface here means the mean geometric line followed by the corners formed by the said groove on the said surface.
A sipe here, means a thin slit with a mean width that is small and such that, under certain loading conditions, the walls of material delimiting it may at least partially come into contact with one another when that sipe is in the contact patch.
In the present description, the terms radial or radially are used to indicate a direction which, when considered on the tire, is a direction perpendicular to the axis of rotation of the tire whereas, when considered on a tread alone, it corresponds to the direction of the thickness of the said tread. Moreover, the term circumferential is used to indicate a direction which corresponds to a direction tangential to any circle centered on the axis of rotation of the tire. This same direction corresponds to the longitudinal direction of the tread, the latter being formed in the manner of a flat strip before it is incorporated al the time of manufacture of a tire.
The disadvantage of creating a plurality of grooves in a tread is that it reduces the amount of tread material for a given width of tread and that it therefore reduces the service life of the tire on account of the increase in wear rate.
Moreover, the grooves reduce the compression and shear stiffnesses because these grooves delimit portions of material that are more sensitive to deformation by comparison with the portions delimited by sipes. Indeed in the case of sipes, the walls of material delimiting these sipes can come into contact with one another at least when the sipe is in the contact patch. This reduction in stiffness, in the case of the presence of grooves, leads to an increase in deformation and generates a reduction in wear performance of the tread: greater wear is observed for a fixed distance covered (this corresponding to an increase in the wear rate of the tread). Moreover, an increase in rolling resistance and therefore in the fuel consumption of vehicles fitted with such tires is observed, this being the result of an increase in hysteresis losses associated with the cycles of deformation of the material of which the tread is made.
Whatever the category of tire (namely whether is be a tire to be fitted to a passenger vehicle or a vehicle intended to carry heavy loads), the tread needs to have standing-water drainage performance that always remains above a minimum performance referred to as the safe performance. Accordingly, and given that the tread gradually wears away, progressively reducing the cross sections of the grooves, it is commonplace to produce grooves that open onto the tread surface when new and that extend into the thickness of the tread down to at least a level that corresponds to a limit that requires the tread to be withdrawn. This has the disadvantage of very appreciably reducing the stiffnesses of the said tread (both the compression stiffness and the shear stiffnesses).
The document published under number WO 2011/039194 has proposed forming in a tread grooves that have the particular feature of opening discontinuously onto the tread surface when new. This groove can be considered to be wavy in the thickness of the tread opening regularly onto the tread surface. This groove comprises a succession of open parts (open cavities), which means to say parts that open onto the tread surface when new, and of closed parts (closed cavities) which means to say ones that do not open directly onto this tread surface when new. These latter so-called closed parts are intended to form new grooves that open onto a tread surface when the tread has become part-worn, A wavy groove may be formed in the circumferential direction or in any other direction in a tread. As described in document WO 2011/039194, a wavy groove is continuous in the initial state (corresponding to the as-new condition of the tread after molding), this making it possible, when driving over a roadway covered with water, to ensure that water is picked up by the groove in the contact patch, the water thus picked up flowing along the wavy groove beneath the tread surface. That same document describes the possibility, after the manner of tread pattern designs of the prior art, to make connections between at least two wavy grooves.